This invention relates to a speed changing apparatus, especially a non-sparking circuit apparatus for a multi-speed fan.
There are two types of speed changing apparatus available on the market. One of them achieves the speed changing effect by changing the starting capacitance of the motor and the other by changing the voltage dropping capacitance the motor. For reducing the costs of the capacitors used in these two types of speed changing apparatus, selector switches have been utilized to connect the motor for the fan with two capacitors in parallel to a single capacitor so that a minimum number of capacitors are required. However, in changing the position of the selector switch, the electrical energy can be discharged from a charged capacitor to another one through the selector switch. This discharging effect can result in damage to the capacitors themselves and to the contacts of the selector switch.
The discharge can occur on the following two conditions: (1.) Upon condition that the two capacitors in the circuit of a fan are electrically connected with the power source at a small time differential, one capacitor is charged prior to the other one. In case the circuit involves A.C. of 50 to 60 Hz, the charges in the earlier charged capacitor can be discharged through the switch to the later charged capacitor. In case the ends of the time differential respectively correspond to the positive and negative peaks of the A.C. wave, the discharge would be significant and may form a spark and the resistance to voltage of the capacitors has to be high enough; (2.) Upon condition that one of the two charged parallel capacitors is disconnected from the power source, there will be residue charges on the disconnected capacitor and the phase of the other capacitor will be changed in a very short time. Thereby, the voltage difference between the two capacitors will be increased. In case the time of the disconnection corresponds to the positive peak of the A.C. wave, a significant discharge and a spark can occur between the two capacitors.